This invention relates to a micropulse laser radar system, and in particular to such a system for use in automatic remote sensing and measurement of atmospheric aerosol concentration and cloud height.
Laser radar systems, and in particular lidar (light detection and ranging) systems have a number of applications including atmospheric monitoring and measurements of cloud height and aerosol concentration. In such systems a laser pulse is transmitted to an atmospheric target, and back-scattered light from clouds and atmospheric aerosol concentrations is collected by a receiver and analysed to derive quantitative atmospheric parameters.
At present, conventional lidar systems usually employ pulse lasers with a relatively high pulse energy of 0.1 to 1 Joule and low pulse repetition rate of several tens of Hertz. However, this high pulse energy presents serious safety problems with regards to ground personnel and indeed potential safety problems with aircraft. It is therefore desirable to provide atmospheric lidar systems with a comparatively low pulse energy and a high pulse repetition rate. Such low-pulse energy systems, however, imply that to be effective the system must have high efficiency to be able to achieve reliable and accurate measurements.
FIG. 2 shows a lidar system according to the prior art as described in James D. Spinhirne, xe2x80x9cMicro Pulse Lidarxe2x80x9d, IEEE Transactions on Geoscience and Remote Sensing, 1993, 31(1). pp48-55. As can be seen in FIG. 2, this system has a bipartite design with the optical transmitter and receiver units placed apart from each other. In this design a frequency doubled 532 nm laser beam from a diode pumped Nd:YLF laser is expanded and collimated before being transmitted to the atmospheric target. The back-scattered signal is collected by a Cassegrain telescope, filtered by a narrow bandwidth interference filter, then focused and directed to a Geiger avalanche photodiode detector. The photon counting signal is then acquired and stored in a personal computer for analysis.
A disadvantage of this system is that it is non-coaxial and requires independent optical transmitter and receiver units, which will increase the volume and weight of the apparatus, thus reducing its mobility. Mobility is an important practical aspect to such systems if they are to be able to produce useful experimental measurements over a wide geographic area. Furthermore, there will be an inevitable mismatch between the field-of-view of the optical transmitter and the field-of-view of the optical receiver. The spatial coupling area will change with field-of-view and detection distance and this will increase the complexity of the subsequent data processing when analyzing the data.
FIG. 3 shows another form of micropulse lidar system as described in I. H. Hwang, Sandor Lokos and Jan Kim, xe2x80x9cMicro Pulse Lidar for Aerosol and Cloud Measurementxe2x80x9d Proc. SPIE on Lidar Atmospheric Monitoring. 1997, Vol.3104, pp39-42. As shown in FIG. 3, this design is an integral system in which a Schmidt-Cassegrain telescope acts as both the optical transmitter and optical receiver. The laser beam from a frequency doubled 532 nm diode pumped Nd:YLF laser is coupled into the telescope, reflected by the second convex mirror and the primary concave mirror, and then transmitted to the atmospheric target. The backscattering return signal is then collected by the same telescope, filtered by an interference filter, focused by an optical lens, detected by an avalanche photodiode and finally captured by a multi-signal capture card for processing by a computer to obtain desired atmospheric information.
By using an integral coaxial transmitter and receiver, the drawbacks of the Spinhirne system are overcome. However, this system has its own disadvantages. Notably it is of a low energy efficiency. This is because the laser beam has a Gaussian distribution with a high intensity in the center of the beam, and low intensity at the edges of the beam. Unfortunately, however, this high-intensity central region is blocked by the second mirror of the Schmidt-Cassegrain telescope and does not contribute to the final transmitted pulses. This amounts to an energy loss of about 30-40%. In addition, the high-intensity central region which is blocked by the second mirror will be returned along the original light path and will generate strong background noise at the detector which will both increase the difficulty of detecting the backscattering signals, and in the long run will damage the detector.
According to the present invention there is provided a micropulse lidar system comprising, a laser light source for emitting a pulsed laser beam, beam shaping means for shaping the intensity distribution of said beam such that said beam has a substantially annular intensity distribution in cross-section, a Schmidt-Cassegrain telescope for transmitting the annular beam to an atmospheric target and for collecting backscattered light returned from said target, and means for detecting and analyzing said backscattered light.
Preferably the micropulse lidar system may further comprise beam expanding and collimating means located between said laser light source and said beam shaping means.
In a preferred embodiment of the invention the lidar system is one with a high pulse repetition rate and low pulse energy. For example the laser light source may emit pulses with a repetition rate of between 1 to 20 KHz and a pulse energy in the range of 1 to 50 xcexcJ. For example the laser light source may be a Nd:YAG laser that emits light at 532 nm.
Viewed from another broad aspect the present invention provides a method for designing a diffractive optical element comprising optimizing the phase distribution of the diffractive optical element using a marginal phase correction method.